Observations of molecules in the Galaxy, whether by absorption, spontaneous emission or maser action, have shown that for all that molecular spectroscopy has long been studied in great detail, there are large gaps in our knowledge of even simple molecules, both in general and in detail. At the most basic level, a number of unidentified radio frequency lines were detected some while ago, and with the extension of observations into the millimetre wave region, many more unidentified lines have been found. Despite earlier extensive work at the National Bureau of Standards of the U. S. A, there remains a great need for measurements of frequencies of transitions in many molecules. The fact that galactic concentrations of molecules are often associated with strong sources of far infrared or ultraviolet radiation has shown that there are many gaps in our knowledge of molecular spectra in those technically difficult ranges of the spectrum and has led to attempts to develop improved techniques for studying them, as well as to interest in some theoretical problems.